Image boot ability verfication and futher actions

Sub Question 1: Could it be considered a bug, when system creates image it can't operate further?

This cannot be Glance's responsibility as what you can boot from is vendor and operator specific things. There is a effort in Glance to add scripts that will enable operators add information on the image to make it boot-able in that deployment. But there cannot be any guarantee on that, also the boot-ablity testing is a NP complete problem to address in any particular one service. The deal is with the user and operator on how to ease the pain point on this.

Sub Question 2: May that be advisable to create special commands in glanceclient which either can change state of image or can safely delete image entry in database as a whole?

Image state is a internal to system so, it shouldn't be changed by the user. Hence, there's no functionality for it.
Image DB should not be accessible to the user, as far as the user is concerned they should not know if Glance has a DB or not.